1. Field of the Invention
This invention relates to a printed wiring board for use to mount a surface mounted part and a fabrication method for a printed wiring board, and more particularly to a printed wiring board and a fabrication method for a printed wiring board suitably applied where patterns of wiring conductors and an insulating material are formed by printing.
2. Description of the Related Art
In recent years, together with reduction in scale and enhancement in function of electronic apparatus, also reduction in scale and increase in mounting density of a printed wiring board built in an electronic apparatus have and are being proceeded.
Also the main current of the size of parts (surface mounted parts) mounted on a printed wiring board is being changed from the 2012 size (2.0 mm long, 1.2 mm wide) or the 1608 size (1.6 mm long, 0.8 mm wide) to the 1005 size (1.00 mm long, 0.5 mm wide) or the 0603 size (0.6 mm long, 0.3 mm wide).
As reduction of the size of parts to be mounted decreases in this manner, also reduction in scale of conductor patterns of wiring lines and so forth and reduction in scale of pads (soldering lands) for soldering parts proceed.
Together with this, also the method of forming a conductor pattern for a wiring line or a solder resist pattern for a surface protective film or the like on a printed wiring board is changing. In particular, the method is changing from a printing method by screen printing which has been used popularly to a photolithography method by which a finer pattern can be formed.
According to the printing method by screen printing, an error of approximately 0.1 mm remains after formation of a pattern. On the other hand, according to the photolithography method, the error after pattern formation can be reduced to ±0.05 mm or less, and therefore, a finer pattern can be formed.
A process of forming solder resist on a printed circuit board by a printing method by screen printing is performed in the following manner.
First, a silk screen on which a pattern of solder resist is formed is used to perform screen printing on a printed wiring board on which a conductor pattern is formed to print ink of the solder resist on the printed circuit board.
Then, irradiation of ultraviolet (UV) rays or heating is performed for the printed wiring board on which the pattern is printed to harden the ink of the solder resist.
Where a pattern of solder resist on a printed wiring board is formed by a printing method as described above, in order to eliminate a possible influence of the error after pattern formation described above, the following countermeasure is taken frequently. In particular, an opening of solder resist to be formed on a soldering land is designed so as to have an area greater or smaller than that of the soldering land in advance. The countermeasure described is disclosed, for example, in Japanese Patent Laid-Open No. 2003-249747 or Japanese Patent Laid-Open No. 2003-31937.
Exemplary patterns of a printed wiring board where a surface mounted part on the printed wiring board is mounted are shown in FIGS. 10A and 10B.
Referring to FIGS. 10A and 10B, a conductor pattern 52 including a wiring line is connected to a soldering land 51, and a pattern of solder resist 54 is formed leaving an opening 53.
In FIGS. 10A and 10B, the opening 53 of the solder resist 54 is designed so as to have an area greater than that of the soldering land 51.
Therefore, a portion (exposed portion) 52a of the conductor pattern 52 is exposed through the opening 53.
The size of the exposed portion 52a of the conductor pattern 52 which forms a wiring line differs depending upon the connection position of the conductor pattern 52 to the soldering land 51, the number of such connection locations, the width of the wiring line and so forth.
Where the part to be mounted is large, even if the size of the exposed portion 52a of the conductor pattern 52 has a dispersion, this does not make any problem. The limit to the error range in the printing method is approximately 0.1 mm as described hereinabove. Since a soldering land on which a part of the 2012 size is to be mounted has a size of 1.2 mm to 1.4 mm, even the printing method can be used to form the soldering land without any problem.
In contrast, a soldering land for mounting a part of the 1005 size has such a small side as approximately 0.6 mm. Therefore, there is the possibility that the pattern error of 0.1 mm by a printing method may have a bad influence on the quality of soldering.
On the other hand, where the opening of solder resist is designed so as to have an area smaller than that of the soldering land, the solder resist covers also an end edge of the soldering land.
If a soldering land is designed in this manner and formed by a printing method, then the ink of the solder resist is likely to suffer from blurring also from an influence of an offset by the soldering land.
Further, since the mesh of the silk screen is as coarse as #100 to #170, the opening of the solder resist after printed does not exhibit a linear contour and the solder resist is likely to partly blur.
Since a soldering land for mounting a part of the 1005 size has a size as small as approximately 0.6 mm as described hereinabove, there is the possibility that appearance of such blurring as described above may have a bad influence on the quality of soldering.
Thus, if the photolithography method is adopted to form a pattern of solder resist, then since the pattern error can be reduced to 0.05 mm or less, also mounting of a small-sized part is permitted.
Therefore, where a small part of the 1005 size or the 0603 size is to be mounted, the photolithography method is usually used to form a conductor pattern or a pattern of solder resist on a printed wiring board.
However, the photolithography method has a drawback in that it involves a greater number of processing steps than and is inferior in productivity to the printing method.
Therefore, where the photolithography method is used to form a pattern, a high production cost is needed for a printed wiring board.
Further, since parts of the 1005 size are recently used in a mass, the price of parts of the 1005 size drops to such a degree that they can be acquired at a lower price than parts of the 2012 size or the 1608 size.
Therefore, if the photolithography method is used to form a pattern in a popular manner on a printed wiring board for mounted parts of the 1005 size, then since it takes a high production cost, the low price of the parts is less likely to be reflected on the price of an electronic apparatus in which the parts are mounted on the printed wiring board.
Therefore, it is demanded to use a printing method by which a pattern can be formed at a comparatively low production cost also on a printed wiring board on which parts of the 1005 size are to be mounted.
In order to solve the problem described above, an embodiment of the present invention provides a printed wiring board and a production method for a printed wiring board on and by which a pattern can be formed using a printing method and soldering can be performed favorably even where a small part of the 1005 size or less is to be mounted.